The present invention relates to improved redox initiating systems for free radical polymerization.
The present invention also relates to improved cure systems for adhesives.
The present invention also relates to improved two-part acrylic adhesives, which may have both structural and non structural applications.
The present invention more particularly relates to an improved two-part acrylic adhesive incorporating an improved activator.
Two-part acrylic adhesives, otherwise known as reactive acrylics or toughened acrylics, are well-known in the art and commerce as useful, high-performance adhesives. A general description of this class of adhesives is given, for example, by A. Bachmann in Adhesives Age, August 1982, pp. 19-23 (June 1979); and by J. A. Graham, chapter 9 in Adhesives in Manufacturing, ed by G. L. Schneberger, Marcel Decker, 1983.
Key factors in performance of adhesives, include resistance to impact and stress, high adhesion to substrates and resistance to environmental hazards. Of further importance is the rate of cure. It is often desirable that the joint parts, one spread with adhesive and the other coated with activator, cure as quickly as possible after being joined together. It is also desirable that the strength of the joint develop rapidly. Further, the more rapid the cure, the less clamp or fixture time required, and, in general, the greater economic advantage being offered, particularly in high volume manufacturing processes. Still further in the case of the two-part acrylics, the useful lifetime, or the "on part life" of the activator is important. It would be desirable to prime, or apply activator, to a surface in anticipation of later assembly, or mating with the opposing, adhesive-coated surface, well in advance of assembly time in order to permit greater facility in manufacturing operations.
The reactive, or two-part acrylic adhesives, largely consist of (a) a monomer/elastomer solution or a monomer/oligomer/polymer solution, containing a free radical initiator, and (b) an accelerator for the initiator. It is common practice to include from about 5 to about 10 percent by weight of an unsaturated carboxylic acid in the adhesive formulation to accelerate cure time and give improved adhesion. A preferred polymerizable acid is methacrylic acid.
The system is cured by polymerization of the monomer(s) or monomer/oligomer mixtures by free radical mechanisms. The free radicals are generated by reaction of the oxidant material, that is, the initiator or catalyst, and the reductant material, that is, the activator. Typically, therefore, the rate of radical generation which, in turn, determines the rate of polymerization or speed of cure, depends upon the nature of the redox reaction.
An early redox reaction, as disclosed in U.S. Pat. No. 3,994,764 consists of benzoyl peroxide (oxidant) and dimethylaniline (reductant). Because of the toxicity of dimethylaniline, a variety of other aromatic amine derivative have been disclosed. In some cases the odor of aromatic amine is unacceptable to the end user. However, the use of less toxic amine reductants does not eliminate the deficiency of an unstable peroxidic material like benzoyl peroxide (BPO). Instability in the peroxide catalyst leads to premature gelation of the adhesive on storage. In practice, BPO is not included in the adhesive portion of the system. In this case, the activator is contained in the monomer solution and the BPO or catalyst solution is separately applied. A more commonly used oxidant is the more thermally stable cumene hydroperoxide (CHP).
As a suitable reductant or activator to be coupled with CHP, condensation products of amines and aldehydes have been used, see e.g. U.S. Pat. No. 3,890,407. Well known commercial activators of this type include a condensate of butyraldehyde and aniline, sold by the R. T. Vanderbilt Company under the trade name VANAX 808.TM., and a butyraldehyde-butylamine condensate sold under the trade name VANAX 833.TM.. A related material which is similar to VANAX 833.TM. but which contains a small amount of copper salt is sold by the Loctite Corp. under the designation LOCTITE NF.
An important deficiency of the amine-aldehyde condensation products is oxidative instability. A surface coated with such an activator becomes inactive toward a surface covered with the acrylic adhesive within a matter of hours if left exposed to the air. Accordingly, a more stable activator is much desired in industry.
The use of redox systems containing peroxy compounds in conjunction with thioureas is known in the literature. The thioureas may be used with transition metal ion salts to initiate polymerization; however, the reaction does not occur readily at room temperature.
V. A. Loprev et al [J. Poly. Sci., (Polymer Chem. Ed.), 17,3411-12 (1979)] briefly reviews the subject of oxidant-thiourea systems. He supports the conclusion of previous workers that electron-withdrawing groups accelerate the polymerization process, while electron-donating groups retard it.
An earlier paper by H. Narita et al [Makromol. Chem., 178,3217-20 (1977)] deals with transition metal ions and thioureas as redox initiators. He suggests that any substitution of the hydrogen atom on the amino group decreases the reactivity of the thiourea in initiating polymerization.
There are also several U.S. patents which deal with thioureas as reductants or initiators for two component curing systems. U.S. Pat. No. 3,991,008, assigned to The Kendall Company, discloses monosubstituted thioureas in conjunction with various hydroperoxides, give a good cure system with improved color stability.
U.S. Pat. No. 3,970,505, assigned to Loctite discloses poly-substituted thioureas in conjunction with an acid to give good "cure through volume" characteristics for anaerobic adhesive systems.
U.S. Pat. No. 3,870,675 assigned to Sumitomo Chemical Co. discloses a reductant system consisting of a poly-substituted thiourea with dipyridyl amine and a transition metal salt for accelerating the cure of a two component adhesive system.
An earlier U.S. Pat. No. 3,625,930, also assigned to Loctite discloses mercaptobenzothiazole and thiourea-type compounds as reductants. However, the ability of these compounds to cure through a gap is generally limited to about 3 mils (0.003 inch). This information is also cited in U.S. Pat. No. 3,970,505 (see column 7. line 15 FF).
The technology disclosed in patent U.S. Pat. No. 3,970,505 indicates that mono- and disubstituted thiourea are relatively ineffective in initiating the cure of two component adhesive systems. This patent also teaches that modification of the substituent group on the thiourea component has little or no effect on the activity of that component in a redox reaction, in direct contradistinction to the teaching embodied in the present invention. U.S. Pat. No. 3,970,505 further teaches that salicylic acid significantly accelerates the curing rates in the presence of tetramethyl thiourea.